More efficiently parse shas for shaPostProcessor (#16101)

* More efficiently parse shas for shaPostProcessor

The shaPostProcessor currently repeatedly calls git rev-parse --verify on both backends
which is fine if there is only one thing that matches a sha - however if there are
multiple things then this becomes wildly inefficient.

This PR provides functions for both backends which are much faster to use.

Fix #16092

* Add ShaExistCache to RenderContext

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>

modules/markup/renderer.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 	"sync"
 
+	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/setting"
 )
 
@@ -35,13 +36,44 @@ func Init() {
 
 // RenderContext represents a render context
 type RenderContext struct {
-	Ctx         context.Context
-	Filename    string
-	Type        string
-	IsWiki      bool
-	URLPrefix   string
-	Metas       map[string]string
-	DefaultLink string
+	Ctx           context.Context
+	Filename      string
+	Type          string
+	IsWiki        bool
+	URLPrefix     string
+	Metas         map[string]string
+	DefaultLink   string
+	GitRepo       *git.Repository
+	ShaExistCache map[string]bool
+	cancelFn      func()
+}
+
+// Cancel runs any cleanup functions that have been registered for this Ctx
+func (ctx *RenderContext) Cancel() {
+	if ctx == nil {
+		return
+	}
+	ctx.ShaExistCache = map[string]bool{}
+	if ctx.cancelFn == nil {
+		return
+	}
+	ctx.cancelFn()
+}
+
+// AddCancel adds the provided fn as a Cleanup for this Ctx
+func (ctx *RenderContext) AddCancel(fn func()) {
+	if ctx == nil {
+		return
+	}
+	oldCancelFn := ctx.cancelFn
+	if oldCancelFn == nil {
+		ctx.cancelFn = fn
+		return
+	}
+	ctx.cancelFn = func() {
+		defer oldCancelFn()
+		fn()
+	}
 }
 
 // Renderer defines an interface for rendering markup file to HTML